The present invention relates to fuel firing and more particularly to an arrangement for reducing the formation of nitric oxides.
There is a present day growing concern with the immediate and long term problems created by the rapid increase in air pollution resulting from a rise in the industrial civilization level throughout the world. With this concern comes an acute awareness that immediate steps must be taken to reverse this upward trend in pollution and great efforts are now being made by public and private economic sectors to develop measures for preventing potentially polluting particles and gases from being discharged into the atmosphere. One such source of atmospheric pollution is the nitrogen oxides (NO.sub.x) present in the stack emission of fossil fuel fired steam generating units. Nitric oxide (NO) is an invisible, relatively harmless gas. However, after it is discharged from the stack and comes into contact with oxygen, it reacts to form nitrogen dioxide (NO.sub.2) or other oxides of nitrogen collectively referred to as nitric oxides. Nitrogen dioxide is a yellow-brown gas which, in sufficient concentrations is toxic to animal and plant life. It is this gas which may create the visible haze at the stack discharge of a vapor generator.
With the advent of stricter emission controls, manufacurers of fuel burning equipment have been actively seeking techniques for limiting the amount of pollutants which are formed from the combustion of fossil fuel. Such techniques are disclosed in U.S. Pat. Nos. 3,788,796; 3,880,570 and 3,904,349 assigned to the Assignee of the present invention.
Nitric oxide is formed as a result of the reaction of nitrogen and oxygen and may be fuel derived nitric oxide and/or thermal nitric oxide. The former occurs from the reaction of the nitrogen contained in the fuel with the oxygen in the combustion air whereas the latter results from the reaction of the nitrogen and oxygen contained in the combustion air.
The rate at which fuel nitric oxide is formed is principally dependent on the oxygen supply in the ignition zone. No appreciable nitric oxide is produced under a reducing atmosphere; that is, a condition where the level of oxygen in the ignition zone is below that required for a complete burning of the fuel. Under these conditions, the fuel nitrogen compounds are decomposed and will not produce nitric oxide in further stages of air supply within regulated temperature levels.
The rate at which thermal nitric oxide is formed is dependent upon any or a combination of the following variables; (1) flame temperature, (2) residence time of the combustion gases in the high temperature zone and (3) excess oxygen supply. The rate of formation of nitric oxide increases as flame temperature increases. In vapor generators of the type hereunder discussion wherein the combustion of fuel and air may generate flame temperatures in the order of 3,700.degree. F., the time-temperature relationship governing the reaction is such that at flame temperature at or below 2,900.degree. F. no appreciable nitric oxide (NO) is produced, whereas above 2,900.degree. F. the rate of reaction increases rapidly.
Thus, one will recognize from the foregoing discussion that the formation of nitric oxide from fuel nitrogen is inhibited by maintaining a reducing atmosphere, and the formation of nitric oxide from air nitrogen is inhibited by maintaining flame temperature at or below 2,900.degree. F.